In order to determine the structure of the earth's subsurface, seismic disturbances are created at or near the earth's surface and the travel time of resulting seismic waves to reflecting subsurface strata and back is measured. One source for generating these seismic waves is the hydraulic type vibrator. Because the impedances of the hydraulic valve, the hydraulic actuator, and the ground change with frequency, it is necessary to employ a servo loop to ensure that the seismic signal imparted into the ground is representative of the electrical input signal that drives the vibrator.
The time lapse between the time the seismic signal is injected into the earth and the return of the reflected seismic wave to the surface is normally determined by correlating the injected wave form with the return wave form and determining points of maximum correlation. One type of servo control loop that has been developed controls only the phase of the injected seismic signal. The one type of seismic signal that has been injected, using this system, is a swept sine wave in which the frequency changes slowly as a function of time. The difficulty that is encountered with a slowly varying sine wave is that, as the injected signal and the reflected signal are correlated, minor lobes, diminishing in amplitude, extend from the major lobe in both directions on the time shift axis of the correlation curve. The side lobes of the correlation curves produced by reflections from a good reflecting horizon may be of sufficient amplitude to mask the major lobe of a small amplitude correlation signal resulting from reflections from a relatively poor reflecting horizon. Also, if only the phase of the injected signal is controlled by the servo loop, it is not possible to compensate for changes in impedances within the vibratory system and the ground, and the desired amplitude of vibration cannot be maintained. Another servo control system that has been developed utilizes only position feedback from the hydraulic valve and the hydraulic actuator. Because the driving point impedance of the earth is rather large, there is going to be some movement of the reaction mass and cylinder relative to the ground. Feedback taken from the hydraulic actuator, which will indicate only relative movement between the piston and cylinder, cannot compensate for movement of the cylinder, and to the extent that the cylinder does move, the injected seismic signal will not be representative of the electrical input signal.